What encryption algorithm is best for encrypting cookies?

Question

Since this question is rather popular, I thought it useful to give it an update.

Let me emphasise the correct answer as given by AviD to

Answer 1

In addition, I have tried the mcrypt_encrypt and one thing please keep in mind. If you do base64_encode(mcrypt_encrypt(...)).

and then later, you do base64_decode and output the encrypted data (echo). You probably will be screwed and not seeing anything. However, if you do mcrypt_decrypt( ... base64_decode($value) ). You will see the original data.